This attribute is optional, as each data type has a default marshaling behavior. This attribute is only necessary when a given type can be marshaled to multiple types. For example, you can marshal a string to unmanaged code as either a LPStr, a LPWStr, a LPTStr, or a BStr. By default, the common language runtime marshals a string parameter as a BStr to COM methods. You can apply the MarshalAsAttribute attribute to an individual field or parameter to cause that particular string to be marshaled as a LPStr instead of a BStr. The Type Library Exporter (Tlbexp.exe) passes your marshaling preferences to the common language runtime.

Some parameters and return values have different default marshaling behavior when used with COM interop or platform invoke. By default, the runtime marshals a string parameter (and fields in a value type) as a LPStr to a platform invoke method or function. For additional information, see Default Marshaling Behavior.

In most cases, the attribute simply identifies the format of the unmanaged data using the UnmanagedType enumeration, as shown in the following C# signature:

Some UnmanagedType enumeration members require additional information. For example, additional information is needed when the UnmanagedType is LPArray. For a complete description of how to use this attribute with arrays, see Default Marshaling for Arrays.

The Type Library Importer (Tlbimp.exe) also applies this attribute to parameters, fields, and return values to indicate that the data type in the input type library is not the default type for the corresponding managed data type. Tlbimp.exe always applies the MarshalAsAttribute to String and Object types for clarity, regardless of the type specified in the input type library.

Imports System.Runtime.InteropServices
Module Program
'Applied to a parameter.PublicSub M1(<MarshalAs(UnmanagedType.LPWStr)> ByVal msg AsString)
EndSub'Applied to a field within a class.Class MsgText
<MarshalAs(UnmanagedType.LPWStr)> Public msg AsStringEndClass'Applied to a a return value.PublicFunction M2() As <MarshalAs(UnmanagedType.LPWStr)> StringEndFunctionEndModule
//Applied to a parameter.
public void M1 ([MarshalAs(UnmanagedType.LPWStr)]String msg);
//Applied to a field within a class.
class MsgText {
[MarshalAs(UnmanagedType.LPWStr)] publicString msg;
}
//Applied to a return value.
[return: MarshalAs(UnmanagedType.LPWStr)]
publicString GetMessage();